PART7Basic Circuit of Semiconductor

Experiment 9 :Characteristic of SCR

Theory

In normal switching TR, to lower the internal resistance of ON status, the impurity concentration of the collector should be raised, and then the reverse voltage of collector becomes lower. To overcome this, a switching element of high reverse voltage and high power is developed, and this is SCR(silicon controlled rectifier) of 4-level structure and 3 terminals as in fig.7-37(A). This is also called a thyristor. Pulse current over certain level should flow to the gate of SCR so that forward current can flow from anode to cathode.(Switch ON)

Operation Principle

To interpret the operation of SCR, think as in fig.7-37(b) that two layers in the middle are split and pnp TR and npn TR are contacted.

• When the gate is earthed, Q1 is cutoff. Here, I_CI=I_B2=I_CO1 is too small to turn on Q2. That is, when V_GK=0, SCR is off.
• When + voltage(normal pulse) which is big enough is applied to the gate, Q1 is turned on, and I_C1=I_B2turns on Q2, and I_C1=I_B2turns on Q1 more. That is, regeneration occurs within Q1 roof so two TRs are turned on. From the moment the voltage is applied to the gate to the moment SCR is on is called turn ON time of SCR, and usually is 0.1 ~ 1μs.
• Once SCR is turned on, the ON status is maintained after removing gate voltage. Tu turn off SCR, Operate anode voltage as 0 or -. The turn OFF time is 1~30μs.
• Once the gate current flows, value of α increases and when α1+α2 reaches 1, anode current I becomes very big. That is, SCR is turned on. If gate current is removed in this status, SCR is still turned on.

Characteristic

Fig.7-38(a) describes the change of anode current when gate current is maintained as 0 and anode voltage is changed. When the anode voltage is increased in - direction, avalanche breakdown of SCR occurs. When it is increased in + direction, α₁+α₂ is small in OA region and SCR operates just as the reverse biased junction. In AB region, α value increases and at breakover voltage, α₁+α₂=1. If the current increases more, SCR operates just as forward biased junction. That is, it operates in CD region where anode voltage is fallen radically. When the hold current corresponding point C is decreased below I_H, the operation point moves from C to the origin 0 and SCR is off.

When current I_G flows at the gate, α value increases and breakover voltage becomes low(fig.7-38(b)). Generally, to turn on, the anode voltage lower than breakover voltage of I_G=0 is applied and trigger signal is applied to the gate. Short pulse with peak voltage of several V, peak current of several mA ~ several tens mA is used as the trigger signal.

Experiment Process

1. To experiment the characteristic of SCR, make a connection as in fig.7-39 using Block d of M07 and supply the power of DC 10V.

2. Change the gate voltage as given in table 7-14 and measure the gate current(IG), the voltage between both ends of load resistance(VO) and load current(IL) and record the result in the relevant column. And decrease the input voltage slowly and record the current at the moment it is turned off(Turn-Off Current).

tab1 print